Light-emitting element comprising stacked light-emitting layers, light-emitting device, electronic device, and lighting device

ABSTRACT

A light-emitting element with a high current efficiency is provided. A low-power consumption light-emitting device is also provided. In addition, low-power consumption electronic device and lighting device are provided. The light-emitting element includes an EL layer between a pair of electrodes. The EL layer includes a light-emitting layer. The light-emitting layer includes a first light-emitting layer and a second light-emitting layer. The emission peak of the second light-emitting layer is at a shorter wavelength than that of the first light-emitting layer. The first light-emitting layer includes a host material and a guest material. The LUMO level of the guest material is in the range of ±0.1 eV of the LUMO level of the host material.

This application is a continuation of copending U.S. application Ser.No. 14/623,928, filed on Feb. 17, 2015 which is incorporated herein byreference.

TECHNICAL FIELD

One embodiment of the present invention relates to a light-emittingelement in which an organic compound capable of emitting light byapplication of an electric field is provided between a pair ofelectrodes, and also relates to a light-emitting device, an electronicdevice, and a lighting device including such a light-emitting element.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, one embodimentof the present invention relates to a process, a machine, manufacture,or a composition of matter. Specifically, examples of the technicalfield of one embodiment of the present invention disclosed in thisspecification include a semiconductor device, a display device, a liquidcrystal display device, a light-emitting device, a lighting device, apower storage device, a memory device, a method for driving any of them,and a method for manufacturing any of them.

BACKGROUND ART

A light-emitting element using an organic compound as a luminous body,which has features such as thinness, lightness, high-speed response, andDC drive at low voltage, is expected to be used in a next-generationflat panel display. In particular, a display device in whichlight-emitting elements are arranged in matrix is considered to haveadvantages in a wide viewing angle and excellent visibility over aconventional liquid crystal display device.

The light emission mechanism is said to be as follows: when a voltage isapplied between a pair of electrodes with an EL layer including aluminous body provided therebetween, electrons injected from the cathodeand holes injected from the anode recombine in the light emission centerof the EL layer to form molecular excitons, and energy is released andlight is emitted when the molecular excitons return to the ground state.Singlet excitation and triplet excitation are known as excited states,and it is thought that light emission can be achieved through either ofthe excited states.

In order to improve the element characteristics of such light-emittingelements, improvement of an element structure, development of materials,and the like have been actively carried out (for example, see PatentDocument 1).

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2010-182699

DISCLOSURE OF INVENTION

The current efficiency of a light-emitting element is one of theimportant evaluation factors in development. Higher current efficiencyreduces power consumption, promising to enable mass production. Alight-emitting element with a high current efficiency therefore needs tobe developed.

One embodiment of the present invention provides a light-emittingelement with a high current efficiency. Another embodiment of thepresent invention provides a low-power consumption light-emitting deviceincluding the light-emitting element. Another embodiment of the presentinvention provides low-power consumption electronic device and lightingdevice. Another embodiment of the present invention provides a novellight-emitting element, a novel light-emitting device, a novel lightingdevice, and the like. Note that the description of these objects doesnot disturb the existence of other objects. Note that in one embodimentof the present invention, there is no need to achieve all the objects.Note that other objects will be apparent from the description of thespecification, the drawings, the claims, and the like and other objectscan be derived from the description of the specification, the drawings,the claims, and the like.

One embodiment of the present invention is a light-emitting elementincluding an EL layer between a pair of electrodes (an anode and acathode). The EL layer includes a light-emitting layer. Thelight-emitting layer includes a first light-emitting layer and a secondlight-emitting layer. The first light-emitting layer is provided betweenthe cathode and the second light-emitting layer. The secondlight-emitting layer is provided between the first light-emitting layerand the anode. The first light-emitting layer includes a region incontact with the second light-emitting layer. The emission peak of thesecond light-emitting layer is at a shorter wavelength than that of thefirst light-emitting layer. The first light-emitting layer includes ahost material and a guest material. The LUMO (Lowest UnoccupiedMolecular Orbital) level of the guest material is in the range of ±0.1eV of the LUMO level of the host material.

Another embodiment of the present invention is a light-emitting elementincluding a first EL layer, a second EL layer, and a charge-generationlayer. The first EL layer, the second EL layer, and thecharge-generation layer are provided between a cathode and an anode. Thecharge-generation layer is provided between the first EL layer and thesecond EL layer. The first EL layer includes a third light-emittinglayer, and the second EL layer includes a fourth light-emitting layer.At least one of the third light-emitting layer and the fourthlight-emitting layer includes a first light-emitting layer and a secondlight-emitting layer. The first light-emitting layer is provided betweenthe cathode and the second light-emitting layer. The secondlight-emitting layer is provided between the first light-emitting layerand the anode. The emission peak of the second light-emitting layer isat a shorter wavelength than that of the first light-emitting layer. Thefirst light-emitting layer includes a host material and a guestmaterial. The LUMO level of the guest material is in the range of ±0.1eV of the LUMO level of the host material.

Another embodiment of the present invention is a light-emitting elementincluding an EL layer between a pair of electrodes (an anode and acathode). The EL layer includes a light-emitting layer. Thelight-emitting layer includes a first light-emitting layer and a secondlight-emitting layer. The first light-emitting layer is provided betweenthe cathode and the second light-emitting layer. The secondlight-emitting layer is provided between the first light-emitting layerand the anode. The first light-emitting layer includes a region incontact with the second light-emitting layer. The emission peak of thesecond light-emitting layer is at a shorter wavelength than that of thefirst light-emitting layer. The first light-emitting layer and thesecond light-emitting layer include the same host material. The LUMOlevel of a guest material included in the first light-emitting layer isin the range of ±0.1 eV of the LUMO level of the host material.

Another embodiment of the present invention is a light-emitting elementincluding a first EL layer, a second EL layer, and a charge-generationlayer. The first EL layer, the second EL layer, and thecharge-generation layer are provided between a cathode and an anode. Thecharge-generation layer is provided between the first EL layer and thesecond EL layer. The first EL layer includes a third light-emittinglayer, and the second EL layer includes a fourth light-emitting layer.At least one of the third light-emitting layer and the fourthlight-emitting layer includes a first light-emitting layer and a secondlight-emitting layer. The first light-emitting layer is provided betweenthe cathode and the second light-emitting layer. The secondlight-emitting layer is provided between the first light-emitting layerand the anode. The emission peak of the second light-emitting layer isat a shorter wavelength than that of the first light-emitting layer. Thefirst light-emitting layer and the second light-emitting layer includethe same host material. The LUMO level of a guest material included inthe first light-emitting layer is in the range of ±0.1 eV of the LUMOlevel of the host material.

In each of the aforementioned structures, the light-emitting elementfurther includes a fifth light-emitting layer. The fifth light-emittinglayer is provided between the first light-emitting layer and thecathode. The fifth light-emitting layer includes a region in contactwith the first light-emitting layer. The fifth light-emitting layer andthe second light-emitting layer include the same material.

In each of the aforementioned structures, the emission in the firstlight-emitting layer has a peak at a wavelength of 560 nm to 700 nm, andthe emission in the second light-emitting layer has a peak at awavelength of 500 nm to 560 nm.

In each of the aforementioned structures, the guest material included inthe first light-emitting layer is a phosphorescent organometalliciridium complex.

Another embodiment of the present invention is a light-emitting deviceincluding the light-emitting element having any of the aforementionedstructures.

The category of one embodiment of the present invention includes notonly a light-emitting device including the light-emitting element butalso an electronic device and a lighting device each including thelight-emitting device. The light-emitting device in this specificationtherefore refers to an image display device or a light source (e.g., alighting device). In addition, the light-emitting device might includeany of the following modules in its category: a module in which aconnector such as a flexible printed circuit (FPC) or a tape carrierpackage (TCP) is attached to a light-emitting device; a module having aTCP provided with a printed wiring board at the end thereof; and amodule having an integrated circuit (IC) directly mounted on alight-emitting element by a chip on glass (COG) method.

According to one embodiment of the present invention, a light-emittingelement with a high current efficiency can be provided. According toanother embodiment of the present invention, a low-power consumptionlight-emitting device including the light-emitting element can beprovided. According to another embodiment of the present invention,low-power consumption electronic device and lighting device can beprovided. According to another embodiment of the present invention, anovel light-emitting element, a novel light-emitting device, a novellighting device, and the like can be provided. Note that the descriptionof these effects does not disturb the existence of other effects. Oneembodiment of the present invention does not necessarily achieve all theabove effects. Other effects will be apparent from and can be derivedfrom the description of the specification, the drawings, the claims, andthe like.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 illustrates a structure of a light-emitting element of oneembodiment of the present invention;

FIGS. 2A and 2B each illustrate a structure of a light-emitting element;

FIG. 3 illustrates a structure of a light-emitting element;

FIGS. 4A and 4B illustrate a structure of a light-emitting device;

FIGS. 5A to 5D each illustrate an electronic device and FIGS. 5D′1 to5D′2 each illustrate an electronic device;

FIG. 6 illustrates lighting devices;

FIG. 7 illustrates structures of a light-emitting element 1, alight-emitting element 2, a comparative light-emitting element 3, and acomparative light-emitting element 4;

FIG. 8 shows luminance-current efficiency characteristics of thelight-emitting element 1, the light-emitting element 2, the comparativelight-emitting element 3, and the comparative light-emitting element 4;and

FIG. 9 shows emission spectra of the light-emitting element 1, thelight-emitting element 2, the comparative light-emitting element 3, andthe comparative light-emitting element 4.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described below in detailwith reference to the drawings. Note that the present invention is notlimited to the following description, and various changes andmodifications can be made without departing from the spirit and scope ofthe present invention. Therefore, the present invention should not/beconstrued as being limited to the description in the followingembodiments.

Embodiment 1

In this embodiment, a light-emitting element of one embodiment of thepresent invention will be described.

In the light-emitting element of one embodiment of the presentinvention, an EL layer including a light-emitting layer is providedbetween a pair of electrodes. The light-emitting layer has a stackedstructure of a first light-emitting layer including at least a guestmaterial (a light-emitting material) and a host material (anelectron-transport material or a hole-transport material) and a secondlight-emitting layer including at least a guest material (alight-emitting material) and a host material (an electron-transportmaterial or a hole-transport material).

The structure of the light-emitting element of one embodiment of thepresent invention will be described below with reference to FIG. 1.

In the light-emitting element illustrated in FIG. 1, an EL layer 103that includes a light-emitting layer 106 is provided between a pair ofelectrodes (an anode 101 and a cathode 102). The EL layer 103 has astructure in which a hole-injection layer 104, a hole-transport layer105, a light-emitting layer 106, an electron-transport layer 107, anelectron-injection layer 108, and the like are stacked over the anode101 in this order.

Note that the light-emitting layer 106 includes a plurality oflight-emitting layers stacked; in FIG. 1, two light-emitting layers (afirst light-emitting layer 106 a and a second light-emitting layer 106b) are stacked. The first light-emitting layer 106 a includes at least aguest material (a first light-emitting material) 109 a and a hostmaterial 110. The second light-emitting layer 106 b includes at least aguest material (a second light-emitting material) 109 b and the hostmaterial 110.

As for the guest material (the first light-emitting material 109 a andthe second light-emitting material 109 b) included in the light-emittinglayer 106 (the first light-emitting layer 106 a and the secondlight-emitting layer 106 b), the emission peak of the secondlight-emitting material 109 b is at a shorter wavelength than that ofthe first light-emitting material 109 a. In addition, the LUMO level ofthe first light-emitting material 109 a included in the firstlight-emitting layer 106 a is in the range of ±0.1 eV of the LUMO levelof the host material 110 included in the first light-emitting layer 106a.

As the host material 110 included in the light-emitting layer 106, anelectron-transport material with an electron mobility of 10⁻⁶ cm²/Vs orhigher or a hole-transport material with a hole mobility of 10⁻⁶ cm²/Vsor higher is mainly used.

When the light-emitting layer 106 has a structure in which the guestmaterial is dispersed in the host material 110, crystallization of thelight-emitting layer 106 can be suppressed. Furthermore, concentrationquenching due to high concentration of the light-emitting material canbe suppressed to increase the emission efficiency of the light-emittingelement.

Note that the stacked light-emitting layers (106 a and 106 b) preferablycontain the same host material 110; however, different materials canalso be used as long as the light-emitting layers function properly.

In addition, the level of a triplet excitation energy (T1 level) of thehost material 110 is preferably higher than the T1 level of the guestmaterial (109 a and 109 b). This is because, when the T1 level of thehost material 110 is lower than that of the guest material (109 a and109 b), the triplet excitation energy of the light-emitting material(109 a and 109 b), which is to contribute to light emission, is quenchedby the host material 110 and accordingly the emission efficiency isreduced.

Note that in the light-emitting element of one embodiment of the presentinvention illustrated in FIG. 1, the light-emitting layer in the ELlayer has the structure in which the two layers of the firstlight-emitting layer 106 a and the second light-emitting layer 106 b arestacked on the cathode. Moreover, the first light-emitting layer 106 aincludes the host material 110 and the guest material (the firstlight-emitting material 109 a) whose LUMO level is in the range of ±0.1eV of the LUMO level of the host material 110. Hence, the carrier(electron) trapping properties in the first light-emitting layer 106 acan be reduced, which prevents over-expansion of the light-emissionregion and increases the current efficiency. Furthermore, carriers(electrons) that are transferred from the first light-emitting layer 106a because of the aforementioned properties effectively move to thesecond light-emitting layer 106 b in contact with the firstlight-emitting layer 106 a, increasing the current efficiency of thesecond light-emitting layer 106 b.

Also in the light-emitting element of one embodiment of the presentinvention illustrated in FIG. 1, a third light-emitting layer containingthe same material as the second light-emitting layer 106 b is preferablyprovided between the first light-emitting layer 106 a and the cathodeand in contact with the first light-emitting layer 106 a. Such astructure offers a light-emitting element that is unlikely to beaffected by a change in carrier balance over time, i.e., a long-lifelight-emitting element.

Next, a specific example in fabrication of the above light-emittingelement will be described.

As the first electrode (anode) 101 and the second electrode (cathode)102, a metal, an alloy, an electrically conductive compound, a mixturethereof, and the like can be used. Specifically, indium oxide-tin oxide(indium tin oxide), indium oxide-tin oxide containing silicon or siliconoxide, indium oxide-zinc oxide (indium zinc oxide), indium oxidecontaining tungsten oxide and zinc oxide, gold (Au), platinum (Pt),nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe),cobalt (Co), copper (Cu), palladium (Pd), and titanium (Ti) can be used.In addition, an element belonging to Group 1 or Group 2 of the periodictable, that is, an alkali metal such as lithium (Li) or cesium (Cs), analkaline earth metal such as calcium (Ca) or strontium (Sr), magnesium(Mg), an alloy containing such an element (e.g., MgAg or AlLi), a rareearth metal such as europium (Eu) or ytterbium (Yb), an alloy containingsuch an element, graphene, and the like can be used. The first electrode(anode) 101 and the second electrode (cathode) 102 can be formed by, forexample, a sputtering method or an evaporation method (including avacuum evaporation method).

The hole-injection layer 104 injects holes into the light-emitting layer106 through the hole-transport layer 105 with a high hole-transportproperty. The hole-injection layer 104 contains a hole-transportmaterial and an acceptor substance, so that electrons are extracted fromthe hole-transport material by the acceptor substance to generate holesand the holes are injected into the light-emitting layer 106 through thehole-transport layer 105. Note that the hole-transport layer 105 isformed using a hole-transport material.

Specific examples of the hole-transport material, which is used for thehole-injection layer 104 and the hole-transport layer 105, includearomatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(carbazol-9-yl)triphenylamine(abbreviation: TCTA), 4,4′,4″-tris(N,N-diphenyl amino)triphenylamine(abbreviation: TDATA), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (abbreviation: MTDATA),and 4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB);3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1);3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2); and3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1). Other examples include carbazole derivativessuch as 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB), and9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation: CzPA).The substances listed here are mainly ones that have a hole mobility of10⁻⁶ cm²/Vs or higher. Note that any substance other than the substanceslisted here may be used as long as the hole-transport property is higherthan the electron-transport property.

Other examples include high molecular compounds such aspoly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine)(abbreviation: PVTPA),poly[N-(4-{N″-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyemethacrylamide](abbreviation: PTPDMA), andpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:Poly-TPD).

Examples of the acceptor substance that is used for the hole-injectionlayer 104 include oxides of metals belonging to Groups 4 to 8 of theperiodic table. Specifically, molybdenum oxide is particularlypreferable.

The light-emitting layer 106 (the first light-emitting layer 106 a andthe second light-emitting layer 106 b) includes the guest material 109(109 a and 109 b) and the host material 110, each of which has theaforementioned structure.

There is no particular limitation on the material that can be used asthe guest material (light-emitting material or emission centersubstance) (109 a and 109 b) in the light-emitting layer 106 (the firstlight-emitting layer 106 a and the second light-emitting layer 106 b). Alight-emitting material that converts singlet excitation energy intoluminescence or a light-emitting material that converts tripletexcitation energy into luminescence can be used. Note that the emissioncolor of the light-emitting material 109 a has a shorter wavelength thanthe emission color of the light-emitting material 109 b. Examples of thelight-emitting material and the emission center substance are givenbelow.

Examples of the light-emitting material that converts singlet excitationenergy into luminescence include a substance that emits fluorescencesuch as

-   N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine    (abbreviation: YGA2S),    4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine    (abbreviation: YGAPA),    4-(9H-carbazol-9-yl)-4′-(9,10-diphenyl-2-anthryl)triphenylamine    (abbreviation: 2YGAPPA),-   N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine    (abbreviation: PCAPA), perylene, 2,5,8,11-tetra(tert-butyl)perylene    (abbreviation: TBP),    4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine    (abbreviation: PCBAPA),-   N,N″-(2-tert-butylanthracene-9,10-diyldi-4,1-phenylene)bis[N,N′,N′-triphenyl-1,4-phenylenediamine]    (abbreviation: DPABPA),-   N,9-diphenyl-N-[4-(9,10-diphenyl-2-anthryl)phenyl]-9H-carbazol-3-amine    (abbreviation: 2PCAPPA),-   N-[4-(9,10-diphenyl-2-anthryl)phenyl]-N,N′,N′-triphenyl-1,4-phenylenediamine    (abbreviation: 2DPAPPA),-   N,N,N′,N′,N″,N″,N′″,N′″-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetraamine    (abbreviation: DBC1), coumarin 30,-   N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine    (abbreviation: 2PCAPA),-   N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine    (abbreviation: 2PCABPhA),-   N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine    (abbreviation: 2DPAPA),-   N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine    (abbreviation: 2DPABPhA),-   9,10-bis(1,1′-biphenyl-2-yl)-N-[4-(9H-carbazol-9-yephenyl]-N-phenylanthracen-2-amine    (abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine    (abbreviation: DPhAPhA), coumarin 545T, N,N′-diphenylquinacridone    (abbreviation: DPQd), rubrene,    5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation:    BPT),    2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile    (abbreviation: DCM1),-   2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile    (abbreviation: DCM2),-   N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine    (abbreviation: p-mPhTD),    7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine    (abbreviation: p-mPhAFD),-   {2-isopropyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile    (abbreviation: DCJTI),-   {2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile    (abbreviation: DCJTB),-   2-(2,6-bis{2-[4-(dimethylamino)phenyl]etheny}-4H-pyran-4-ylidene)propanedinitrile    (abbreviation: BisDCM), and-   2-{2,6-bis[2-(8-methoxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile    (abbreviation: BisDCJTM).

Furthermore, examples of the light-emitting material converting tripletexcitation energy into luminescence include a substance emittingphosphorescence and a thermally activated delayed fluorescence (TADF)material that exhibits thermally activated delayed fluorescence. Notethat “delayed fluorescence” exhibited by the TADF material refers tolight emission having the same spectrum as normal fluorescence and anextremely long lifetime, i.e., 10⁻⁶ seconds or longer, preferably 10⁻³seconds or longer.

Examples of the substance that emits phosphorescence includebis[2-(3′,5′-bistrifluoromethylphenyl)pyridinato-N,C^(2′)]iridium(III)picolinate (abbreviation: Ir(CF₃ppy)₂(pic)),

-   bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)    acetylacetonate (abbreviation: Flracac),    tris(2-phenylpyridinato)iridium(III) (abbreviation: Ir(ppy)₃),-   bis(2-phenylpyridinato)iridium(III) acetylacetonate (abbreviation:    Ir(ppy)2(acac)),-   tris(acetylacetonato) (monophenanthroline)terbium(III)    (abbreviation: Tb(acac)3(Phen)),-   bis(benzo[h]quinolinato)iridium(III) acetylacetonate (abbreviation:    Ir(bzq)2(acac)),-   bis(2,4-diphenyl-1,3-oxazolato-N,C²′)iridium(III) acetylacetonate    (abbreviation: Ir(dpo)₂(acac)),    bis{2-[4′-(perfluorophenyl)phenyl]pyridinato-N,C^(2′)}iridium(III)    acetylacetonate (abbreviation: Ir(p-PF-ph)₂(acac)),-   bis(2-phenylbenzothiazolato-N, C^(2′))iridium(III) acetylacetonate    (abbreviation: Ir(bt)₂(acac)),    bis[2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C^(3′)]iridium(III)    acetylacetonate (abbreviation: Ir(btp)₂(acac)),-   bis(1-phenylisoquinolinato-N,C^(2′))iridium(III) acetylacetonate    (abbreviation: Ir(piq)₂(acac)),    (acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)    (abbreviation: Ir(Fdpq)₂(acac)),-   (acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III)    (abbreviation: [Ir(mppr-Me)₂(acac)]),-   (acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)    iridium(III) (abbreviation: [Ir(mppr-iPr)₂(acac)]),-   (acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)    (abbreviation: Ir(tppr)₂(acac)), bis(2,3,5-triphenylpyrazinato)    (dipivaloylmethanato)iridium(III) (abbreviation: [Ir(tppr)₂(dpm)],-   (acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)    (abbreviation: [Ir(tBuppm)₂(acac)]),    (acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)    (abbreviation: [Ir(dppm)₂(acac)]),    2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II)    (abbreviation: PtOEP), tris(1,3-diphenyl-1,3-propanedionato)    (monophenanthroline)europium(III) (abbreviation: Eu(DBM)₃(Phen)),    and-   tris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III)    (abbreviation: Eu(TTA)₃(Phen)).

Specific examples of the TADF material include fullerene, a derivativethereof, an acridine derivative such as proflavine, and eosin. Otherexamples include a metal-containing porphyrin, such as a porphyrincontaining magnesium (Mg), zinc (Zn), cadmium (Cd), tin (Sn), platinum(Pt), indium (In), or palladium (Pd). Examples of the metal-containingporphyrin include a protoporphyrin-tin fluoride complex (SnF₂(ProtoIX)), a mesoporphyrin-tin fluoride complex (SnF₂(Meso IX)), ahematoporphyrin-tin fluoride complex (SnF₂(Hemato IX)), a coproporphyrintetramethyl ester-tin fluoride complex (SnF₂(Copro III-4Me)), anoctaethylporphyrin-tin fluoride complex (SnF₂(OEP)), anetioporphyrin-tin fluoride complex (SnF₂(Etio I)), and anoctaethylporphyrin-platinum chloride complex (PtCl₂OEP). Alternatively,a heterocyclic compound including a π-electron rich heteroaromatic ringand a π-electron deficient heteroaromatic ring can be used, such as2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazol-11-yl)-1,3,5-triazine(PIC-TRZ). Note that a material in which the π-electron richheteroaromatic ring is directly bonded to the π-electron deficientheteroaromatic ring is particularly preferably used because both thedonor property of the π-electron rich heteroaromatic ring and theacceptor property of the π-electron deficient heteroaromatic ring areincreased and the energy difference between the S1 level and the T1level becomes small.

As the electron-transport material, which can be used as the hostmaterial 110 in the light-emitting layer 106 (106 a and 106 b), aπ-electron deficient heteroaromatic compound such as anitrogen-containing heteroaromatic compound is preferable, examples ofwhich include quinoxaline derivatives and dibenzoquinoxaline derivativessuch as 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTPDBq-II),2-[3-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),2-[4-(3,6-diphenyl-9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline(abbreviation: 2CzPDBq-III),7-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:7mDBTPDBq-II), and6-[3-(dibenzothlophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:6mDBTPDBq-II). Note that in the case where a quinoxaline derivative isused as the electron-transport host material, the guest material ispreferably an organometallic complex (specifically, iridium complex) inwhich a ligand is a phenylpyrazine derivative having an alkyl group,because the electron-trapping properties are reduced.

As the hole-transport material, which can be used as the host material110 in the light-emitting layer 106 (106 a and 106 b), a π-electron richheteroaromatic compound (e.g., a carbazole derivative or an indolederivative) or an aromatic amine compound is preferable, examples ofwhich include

-   4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:    PCBA1BP),-   4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine    (abbreviation: PCBNBB),    3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole    (abbreviation: PCzPCN1), 4,4′,4″-tris    [N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation:    1′-TNATA),-   2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-spiro-9,9′-bifluorene    (abbreviation: DPA2SF),-   N,N′-bis(9-phenylcarbazol-3-yl)-N,N′-diphenylbenzene-1,3-diamine    (abbreviation: PCA2B),    N-(9,9-dimethyl-2-diphenylamino-9H-fluoren-7-yl)diphenylamine    (abbreviation: DPNF),-   N,N′,N″-triphenyl-N,N′,N″-tris(9-phenylcarbazol-3-yl)benzene-1,3,5-triamine    (abbreviation: PCA3B),-   2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene    (abbreviation: PCASF),    2-[N-(4-diphenylaminophenyl)-N-phenylamino]spiro-9,9′-bifluorene    (abbreviation: DPASF),-   N,N′-bis[4-(carbazol-9-yl)phenyl]-N,N′-diphenyl-9,9-dimethylfluorene-2,7-diamine    (abbreviation: YGA2F),    4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (abbreviation:    TPD), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl    (abbreviation: DPAB),-   N-(9,9-dimethyl-9H-fluoren-2-yl)-N-{9,9-dimethyl-2-[N′-phenyl-N′-(9,9-dimethyl-9H-fluoren-2-yl)amino]-9H-fluoren-7-yl}phenylamine    (abbreviation: DFLADFL),-   3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole    (abbreviation: PCzPCA1),    3-[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole    (abbreviation: PCzDPA1),-   3,6-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole    (abbreviation: PCzDPA2),-   4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl    (abbreviation: DNTPD),-   3,6-bis[N-(4-diphenylaminophenyl)-N-(1-naphthyl)amino]-9-phenylcarbazole    (abbreviation: PCzTPN2), and-   3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole    (abbreviation: PCzPCA2).

Note that the light-emitting layer 106 (106 a and 106 b) may furtherinclude a material other than the host material and the guest material.For example, the light-emitting layer 106 (106 a and 106 b) preferablyincludes the aforementioned electron-transport material as the hostmaterial, and further includes the aforementioned hole-transportmaterial.

The electron-transport layer 107 is a layer that contains a substancehaving a high electron-transport property. For the electron-transportlayer 107, it is possible to use a metal complex such as Alq₃,tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq₂), BAlq,Zn(BOX)₂, or bis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation:Zn(BTZ)₂). A heteroaromatic compound such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis [5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or4,4′-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs) can alsobe used. A high molecular compound such as poly(2,5-pyridinediyl)(abbreviation: PPy),poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py) orpoly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy) can also be used. The substances listed here aremainly ones that have an electron mobility of 10⁻⁶ cm²/Vs or higher.Note that any substance other than the substances listed here may beused for the electron-transport layer 107 as long as theelectron-transport property is higher than the hole-transport property.

The electron-transport layer 107 is not limited to a single layer, butmay be a stack of two or more layers each containing any of thesubstances listed above.

The electron-injection layer 108 is a layer that contains a substancehaving a high electron-injection property. For the electron-injectionlayer 108, an alkali metal, an alkaline earth metal, or a compoundthereof such as lithium fluoride (LiF), cesium fluoride (CsF), calciumfluoride (CaF₂), or lithium oxide (LiO_(x)) can be used. A rare earthmetal compound like erbium fluoride (ErF₃) can also be used. Anelectride may also be used for the electron-injection layer 108.Examples of the electride include a substance in which electrons areadded at high concentration to calcium oxide-aluminum oxide. Any of thesubstances for forming the electron-transport layer 107, which arelisted above, can also be used.

A composite material in which an organic compound and an electron donor(donor) are mixed may also be used for the electron-injection layer 108.The composite material is superior in an electron-injection property andan electron-transport property, since electrons are generated in theorganic compound by the electron donor. In this case, the organiccompound is preferably a material that is excellent in transporting thegenerated electrons. Specifically, for example, the substances forforming the electron-transport layer 107 (e.g., a metal complex or aheteroaromatic compound), which are given above, can be used. As theelectron donor, a substance showing an electron-donating property withrespect to the organic compound may be used. Specifically, an alkalimetal, an alkaline earth metal, and a rare earth metal are preferable,and lithium, cesium, magnesium, calcium, erbium, ytterbium, and the likeare given. In addition, an alkali metal oxide or an alkaline earth metaloxide is preferable, and lithium oxide, calcium oxide, and barium oxideare given. A Lewis base such as magnesium oxide can also be used. Anorganic compound such as tetrathiafulvalene (abbreviation: TTF) can alsobe used.

Note that each of the hole-injection layer 104, the hole-transport layer105, the light-emitting layer 106 (106 a and 106 b), theelectron-transport layer 107, and the electron-injection layer 108 canbe formed by a method such as an evaporation method (e.g., a vacuumevaporation method), an ink jet method, or a coating method.

In the above-described light-emitting element, carriers are injectedbecause of a potential difference generated between the first electrode101 and the second electrode 102, and the holes and the electrons arerecombined in the EL layer 103, whereby light is emitted. Then, theemitted light is extracted outside through one or both of the firstelectrode 101 and the second electrode 102. Thus, one or both of thefirst electrode 101 and the second electrode 102 are electrodes havinglight-transmitting properties.

A light-emitting element having the structure described in thisembodiment can emit light with plural kinds of colors, particularly emita kind of color light with a high efficiency, which increases thecurrent efficiency of the other emission colors and also increases theemission efficiency of the whole light-emitting element.

The structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 2

In this embodiment, as one embodiment of the present invention, alight-emitting element (hereinafter referred to as a tandemlight-emitting element) that includes a plurality of EL layers with acharge-generation layer provided therebetween will be described withreference to FIGS. 2A and 2B.

A light-emitting element described in this embodiment is a tandemlight-emitting element including a plurality of EL layers (a first ELlayer 202(1) and a second EL layer 202(2)) between a pair of electrodes(a first electrode 201 and a second electrode 204) as illustrated inFIG. 2A.

In this embodiment, the first electrode 201 functions as an anode, andthe second electrode 204 functions as a cathode. Note that the firstelectrode 201 and the second electrode 204 can have structures similarto those described in Embodiment 1. In addition, all or any of theplurality of EL layers (the first EL layer 202(1) and the second ELlayer 202(2)) may have structures similar to those described inEmbodiment 1. In other words, the structures of the first EL layer202(1) and the second EL layer 202(2) may be the same or different fromeach other and can be similar to those of the EL layers described inEmbodiment 1.

In addition, a charge-generation layer 205 is provided between theplurality of EL layers (the first EL layer 202(1) and the second ELlayer 202(2)). The charge-generation layer 205 has a function ofinjecting electrons into one of the EL layers and injecting holes intothe other of the EL layers when voltage is applied between the firstelectrode 201 and the second electrode 204. In this embodiment, whenvoltage is applied such that the potential of the first electrode 201 ishigher than that of the second electrode 204, the charge-generationlayer 205 injects electrons into the first EL layer 202(1) and injectsholes into the second EL layer 202(2).

Note that in terms of light extraction efficiency, the charge-generationlayer 205 preferably has a property of transmitting visible light(specifically, the charge-generation layer 205 has a visible lighttransmittance of 40% or more). The charge-generation layer 205 functionsproperly even when it has lower conductivity than the first electrode201 or the second electrode 204.

The charge-generation layer 205 may have either a structure in which anelectron acceptor (acceptor) is added to a hole-transport material or astructure in which an electron donor (donor) is added to anelectron-transport material. Alternatively, both of these structures maybe stacked.

In the case of the structure in which an electron acceptor is added to ahole-transport material, as the hole-transport material, for example, anaromatic amine compound such as NPB, TPD, TDATA, M (DATA, or4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), can be used. The substances listed here are mainlyones that have a hole mobility of 10⁻⁶ cm²/Vs or higher. Note that anysubstance other than the materials listed here may be used as long asthe hole-transport property is higher than the electron-transportproperty.

Examples of the electron acceptor include

-   7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:    F4-TCNQ), and chloranil. Transition metal oxides can also be given.    Oxides of metals belonging to Groups 4 to 8 of the periodic table    can also be used. Specifically, vanadium oxide, niobium oxide,    tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide,    manganese oxide, and rhenium oxide are preferable because of their    high electron-accepting properties. Among these, molybdenum oxide is    especially preferable because it is stable in the air, has a low    hygroscopic property, and is easy to handle.

On the other hand, in the case of the structure in which an electrondonor is added to an electron-transport material, as theelectron-transport material, for example, a metal complex having aquinoline skeleton or a benzoquinoline skeleton, such as Alq, Almq₃,BeBq₂, or BAlq can be used. Alternatively, a metal complex having anoxazole-based ligand or a thiazole-based ligand, such as Zn(BOX)₂ orZn(BTZ)₂ can be used. Further alternatively, in addition to such a metalcomplex, PBD, OXD-7, TAZ, BPhen, BCP, or the like can be used. Thematerials listed here are mainly ones that have an electron mobility of10⁻⁶ cm²/Vs or higher. Note that any substance other than the materialslisted here may be used as long as the electron-transport property ishigher than the hole-transport property.

As the electron donor, it is possible to use an alkali metal, analkaline earth metal, a rare earth metal, metals belonging to Groups 2and 13 of the periodic table, or an oxide or carbonate thereof.Specifically, lithium (Li), cesium (Cs), magnesium (Mg), calcium (Ca),ytterbium (Yb), indium (In), lithium oxide, cesium carbonate, or thelike is preferably used. Alternatively, an organic compound such astetrathianaphthacene may be used as the electron donor.

Note that forming the charge-generation layer 205 by using any of theabove materials can suppress a drive voltage increase caused by thestack of the EL layers.

Although the light-emitting element including two EL layers is describedin this embodiment, the present invention can be similarly applied to alight-emitting element in which n EL layers (202(1) to 202(n)) (n isthree or more) are stacked as illustrated in FIG. 2B. In the case wherea plurality of EL layers are included between a pair of electrodes as inthe light-emitting element according to this embodiment, by providingcharge-generation layers (205(1) to 205(n-1)) between the EL layers,light emission in a high luminance region can be obtained with currentdensity kept low. Since the current density can be kept low, the elementcan have a long lifetime. When the light-emitting element is applied tolighting, voltage drop due to resistance of an electrode material can bereduced, which results in homogeneous light emission in a large area. Inaddition, a low-power-consumption light-emitting device that can bedriven at low voltage can be achieved.

When the EL layers have different emission colors, a desired emissioncolor can be obtained from the whole light-emitting element. Forexample, in the light-emitting element having two EL layers, when anemission color of the first EL layer and an emission color of the secondEL layer are made to be complementary colors, a light-emitting elementemitting white light as a whole light-emitting element can also beobtained. Note that “complementary colors” refer to colors that canproduce an achromatic color when mixed. In other words, when lightobtained from a light-emitting substance and light of a complementarycolor are mixed, white light emission can be obtained.

The same can be applied to a light-emitting element having three ELlayers. For example, the light-emitting element as a whole can providewhite light emission when the emission color of the first EL layer isred (e.g., the emission spectrum has a peak at 580 nm to 680 nm), theemission color of the second EL layer is green (e.g., the emissionspectrum has a peak at 500 nm to 560 nm), and the emission color of thethird EL layer is blue (e.g., the emission spectrum has a peak at 400 nmto 480 nm).

As a light-emitting device including the above-described light-emittingelement, a passive matrix light-emitting device and an active matrixlight-emitting device can be fabricated. It is also possible tofabricate a light-emitting device having a microcavity structure. Eachof the light-emitting devices is one embodiment of the presentinvention.

Note that there is no particular limitation on the structure of thetransistor (FET) in the case of fabricating the active matrixlight-emitting device. For example, a staggered FET or an invertedstaggered FET can be used as appropriate. A driver circuit formed overan FET substrate may be formed using both an n-type FET and a p-type FETor only either an n-type FET or a p-type FET. Furthermore, there is noparticular limitation on the crystallinity of a semiconductor film usedfor the FET. For example, either an amorphous semiconductor film or acrystalline semiconductor film can be used. Examples of a semiconductormaterial include Group 13 semiconductors (e.g., gallium), Group 14semiconductors (e.g., silicon), compound semiconductors (including oxidesemiconductors), and organic semiconductors.

The structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 3

In this embodiment, a light-emitting device of one embodiment of thepresent invention will be described.

A light-emitting device described in this embodiment has a micro opticalresonator (microcavity) structure, which utilizes a light resonanteffect between a pair of electrodes. The light-emitting device includesa plurality of light-emitting elements each of which has at least an ELlayer 305 between a pair of electrodes (a reflective electrode 301 and atransflective electrode 302) as illustrated in FIG. 3. The EL layer 305includes at least a light-emitting layer 304 serving as a light-emittingarea and may further include a hole-injection layer, a hole-transportlayer, an electron-transport layer, an electron-injection layer, acharge-generation layer, and the like.

The light-emitting device described in this embodiment includes twokinds of light-emitting elements (a first light-emitting element 310Pand a second light-emitting element 310Q) as illustrated in FIG. 3.

The first light-emitting element 310P has a structure in which a firsttransparent conductive layer 303 a, the EL layer 305 partly includingthe light-emitting layer 304, and the transflective electrode 302 arestacked in this order over the reflective electrode 301. The secondlight-emitting element 310Q has a structure in which a secondtransparent conductive layer 303 b, the EL layer 305 partly includingthe light-emitting layer 304, and the transflective electrode 302 arestacked in this order over the reflective electrode 301.

The reflective electrode 301, the EL layer 305, and the transflectiveelectrode 302 are common to the two kinds of light-emitting elements inthis embodiment. The light-emitting layer 304 has a stacked structure ofa layer that emits light (λ_(P)) having a peak in a first wavelengthregion and a layer that emits light (λ_(Q)) having a peak in a secondwavelength region. In this case, the above wavelengths satisfy therelation of λ_(Q)<λ_(P).

Each of the light-emitting elements has a structure in which the ELlayer 305 is provided between the reflective electrode 301 and thetransflective electrode 302. Light emitted in all directions from thelight-emitting layers included in the EL layer 305 is resonated by thereflective electrode 301 and the transflective electrode 302 whichfunction as a micro optical resonator (a microcavity).

Note that the reflective electrode 301 is formed using a conductivematerial having reflectivity, and is formed to a film having a visiblelight reflectivity of 40% to 100%, preferably 70% to 100%, and aresistivity of 1×10⁻² Ωcm or lower. In addition, the transflectiveelectrode 302 is formed using a conductive material having reflectivityand a conductive material having a light-transmitting property, and isformed to a film having a visible light reflectivity or 20% to 80%,preferably 40% to 70%, and a resistivity of 1×10⁻² Ωcm or lower.

The first transparent conductive layer 303 a and the second transparentconductive layer 303 b are formed to different thicknesses, whereby thetwo kinds of light-emitting elements differ from each other in theoptical distance between the reflective electrode 301 and thetransflective electrode 302. Thus, light with a wavelength that isresonated between the reflective electrode 301 and the transflectiveelectrode 302 can be intensified while light with a wavelength that isnot resonated therebetween can be attenuated, so that light withwavelengths which differ depending on the light-emitting elements can beextracted.

Furthermore, in the first light-emitting element 310P, the totalthickness (total optical thickness) from the reflective electrode 301 tothe transflective electrode 302 is set to mλ_(P)/2 (m is a naturalnumber), and in the second light-emitting element 310Q, the totalthickness from the reflective electrode 301 to the transflectiveelectrode 302 is set to mλ_(Q)/2 (m is a natural number).

In this manner, the light (λ_(P)) emitted from the first light-emittinglayer 304P included in the EL layer 305 is mainly extracted from thefirst light-emitting element 310P, and the light (λ_(Q)) emitted fromthe second light-emitting layer 304Q included in the EL layer 305 ismainly extracted from the second light-emitting element 310Q. Note thatthe light extracted from each of the light-emitting elements is emittedfrom the transflective electrode 302 side.

Furthermore, strictly speaking, the total thickness from the reflectiveelectrode 301 to the transflective electrode 302 can be the totalthickness from a reflection region in the reflective electrode 301 to areflection region in the transflective electrode 302. However, it isdifficult to precisely determine the positions of the reflection regionsin the reflective electrode 301 and the transflective electrode 302;therefore, it is presumed that the above effect can be sufficientlyobtained wherever the reflection regions may be set in the reflectiveelectrode 301 and the transflective electrode 302.

Moreover, in the first light-emitting element 310P, the optical distancebetween the reflective electrode 301 and the first light-emitting layer304P is adjusted to a desired thickness ((2m′+1)λ_(P)/4, where m′ is anatural number); thus, light emitted from the first light-emitting layer304P can be amplified.

Note that strictly speaking, the optical distance between the reflectiveelectrode 301 and the first light-emitting layer 304P can be the opticaldistance between a reflection region in the reflective electrode 301 anda light-emitting region in the first light-emitting layer 304P. However,it is difficult to precisely determine the positions of the reflectionregion in the reflective electrode 301 and the light-emitting region inthe first light-emitting layer 304P; therefore, it is presumed that theabove effect can be sufficiently obtained wherever the light-emittingregion may be set in the first light-emitting layer 304P.

Moreover, in the second light-emitting element 310Q, the opticaldistance between the reflective electrode 301 and the secondlight-emitting layer 304Q is adjusted to a desired thickness((2m″+1)λ_(Q)/4, where m″ is a natural number); thus, light emitted fromthe second light-emitting layer 304Q can be amplified.

Note that strictly speaking, the optical distance between the reflectiveelectrode 301 and the second light-emitting layer 304Q can be theoptical distance between a reflection region in the reflective electrode301 and a light-emitting region in the second light-emitting layer 304Q.However, it is difficult to precisely determine the positions of thereflection region in the reflective electrode 301 and the light-emittingregion in the second light-emitting layer 304Q; therefore, it ispresumed that the above effect can be sufficiently obtained wherever thelight-emitting region may be set in the second light-emitting layer304Q.

Note that the light-emitting element in the above-described structureincludes a plurality of light-emitting layers in the EL layer, thepresent invention is not limited thereto; for example, the structure ofthe tandem light-emitting element which is described in Embodiment 2 canbe combined, in which case one light-emitting element includes aplurality of EL layers with a charge-generation layer therebetween andone or more light-emitting layers are formed in each of the EL layers.

The light-emitting device described in this embodiment has a microcavitystructure. Light with wavelengths which differ depending on thelight-emitting element can be extracted even when they include the sameEL layer, so that it is not needed to form light-emitting elements forplural colors. Therefore, the above structure is advantageous for fullcolor display owing to easiness in achieving higher resolution displayor the like. Note that a combination with coloring layers (colorfilters) is also possible. In addition, emission intensity with apredetermined wavelength in the front direction can be increased,whereby power consumption can be reduced. The above structure isparticularly useful in the case of being applied to a color display(image display device) including pixels of three or more colors but mayalso be applied to lighting or the like.

Embodiment 4

In this embodiment, a light-emitting device including a light-emittingelement of one embodiment of the present invention will be described.

The light-emitting device may be either a passive matrix light-emittingdevice or an active matrix light-emitting device. Note that any of thelight-emitting elements described in the other embodiments can be usedfor the light-emitting device described in this embodiment.

In this embodiment, an active matrix light-emitting device is describedwith reference to FIGS. 4A and 4B.

Note that FIG. 4A is a top view illustrating a light-emitting device andFIG. 4B is a cross-sectional view taken along the chain line A-A′ inFIG. 4A. In the active matrix light-emitting device according to thisembodiment, a pixel portion 402, a driver circuit portion (a source linedriver circuit) 403, and driver circuit portions (gate line drivercircuits) 404 (404 a and 404 b) are provided over an element substrate401. The pixel portion 402, the driver circuit portion 404, and thedriver circuit portions 404 are sealed between the element substrate 401and a sealing substrate 406 with a sealant 405.

In addition, a lead wiring 407 for connecting an external input terminalis provided over the element substrate 401. Through the external inputterminal, a signal (e.g., a video signal, a clock signal, a startsignal, or a reset signal) or a potential is transmitted from theoutside to the driver circuit portion 403 and the driver circuitportions 404. Here, a flexible printed circuit (FPC) 408 is provided asan example of the external input terminal. Although only the FPC isillustrated here, the FPC may be provided with a printed wiring board(PWB). The light-emitting device in this specification includes, in itscategory, not only the light-emitting device itself but also thelight-emitting device provided with the FPC or the PWB.

Next, a cross-sectional structure is described with reference to FIG.4B. The driver circuit portion and the pixel portion are formed over theelement substrate 401; the driver circuit portion 403 that is the sourceline driver circuit and the pixel portion 402 are illustrated here.

As an example of the driver circuit portion 403, an FET 409 and an FET410 are combined. Note that the driver circuit portion 403 may be formedwith a circuit including transistors having the same conductivity type(either an n-channel transistor or a p-channel transistor) or a CMOScircuit including an n-channel transistor and a p-channel transistor. Inthis embodiment, the driver circuit is integrated with the substrate;however, the driver circuit is not necessarily formed over thesubstrate, and may be formed outside the substrate.

The pixel portion 402 includes a plurality of pixels each of whichincludes a switching FET 411, a current control FET 412, and a firstelectrode (anode) 413 that is electrically connected to a wiring (asource electrode or a drain electrode) of the current control FET 412.In this embodiment, the pixel portion 402 includes, but is not limitedto, two FETs, the switching FET 411 and the current control FET 412. Thepixel portion 402 may include, for example, three or more FETs and acapacitor in combination.

As the FETs 409, 410, 411, and 412, for example, a staggered transistoror an inverted staggered transistor can be used. Examples of asemiconductor material that can be used for the FETs 409, 410, 411, and412 include Group 13 semiconductors (e.g., gallium), Group 14semiconductors (e.g., silicon), compound semiconductors, oxidesemiconductors, and organic semiconductors. In addition, there is noparticular limitation on the crystallinity of the semiconductormaterial, and an amorphous semiconductor or a crystalline semiconductorcan be used. In particular, an oxide semiconductor is preferably usedfor the FETs 409, 410, 411, and 412. Examples of the oxide semiconductorinclude an In—Ga oxide and an In—M—Zn oxide (M is Al, Ga, Y, Zr, La, Ce,or Nd). For example, an oxide semiconductor that has an energy gap of 2eV or more, preferably 2.5 eV or more, further preferably 3 eV or moreis used for the FETs 409, 410, 411, and 412, so that the off-statecurrent of the transistors can be reduced.

An insulator 414 is formed to cover end portions of the first electrode413. In this embodiment, the insulator 414 is formed using a positivephotosensitive acrylic resin. The first electrode 413 is used as ananode in this embodiment.

The insulator 414 preferably has a curved surface with curvature at anupper end portion or a lower end portion thereof. This providesfavorable coverage with a film to be formed over the insulator 414. Theinsulator 414 can be formed using, for example, either a negativephotosensitive resin or a positive photosensitive resin. The material ofthe insulator 414 is not limited to an organic compound, and aninorganic compound such as silicon oxide, silicon oxynitride, or siliconnitride can also be used.

An EL layer 415 and a second electrode (cathode) 416 are formed over thefirst electrode (anode) 413. The EL layer 415 includes at least alight-emitting layer that has the stacked structure shown inEmbodiment 1. In addition to the light-emitting layer, a hole-injectionlayer, a hole-transport layer, an electron-transport layer, anelectron-injection layer, a charge-generation layer, and the like can beprovided as appropriate in the EL layer 415.

A light-emitting element 417 is formed with a stack of the firstelectrode (anode) 413, the EL layer 415, and the second electrode(cathode) 416. For the first electrode (anode) 413, the EL layer 415,and the second electrode (cathode) 416, the materials described inEmbodiment 1 can be used. Although not illustrated, the second electrode(cathode) 416 is electrically connected to the FPC 408 which is anexternal input terminal.

Although the cross-sectional view of FIG. 4B illustrates only onelight-emitting element 417, a plurality of light-emitting elements arearranged in matrix in the pixel portion 402. Light-emitting elementswhich provide three kinds of light emission (R, G, and B) areselectively formed in the pixel portion 402, whereby a light-emittingdevice capable of full color display can be fabricated. Other than thelight-emitting element which provides three kinds of light emission (R,G, and B), for example, a light-emitting element that emits white (W),yellow (Y), magenta (M), and cyan (C) light may be formed. When theabove light-emitting element that provides several kinds of lightemission is provided as well as a light-emitting element that providesthree kinds of light emission (R, G, and B), for example, higher colorpurity, lower power consumption, or the like can be achieved.Alternatively, a light-emitting device capable of performing full colordisplay may be provided by a combination with color filters.

Furthermore, the sealing substrate 406 is attached to the elementsubstrate 401 with the sealant 405, whereby a light-emitting element 417is provided in a space 418 surrounded by the element substrate 401, thesealing substrate 406, and the sealant 405. Note that the space 418 maybe filled with an inert gas (such as nitrogen and argon) or the sealant405.

An epoxy-based resin or glass frit is preferably used for the sealant405. The material preferably allows as little moisture and oxygen aspossible to penetrate. As the sealing substrate 406, a glass substrate,a quartz substrate, or a plastic substrate formed of fiber-reinforcedplastic (FRP), poly(vinyl fluoride) (PVF), polyester, acrylic, or thelike can be used. In the case where glass frit is used as the sealant,the element substrate 401 and the sealing substrate 406 are preferablyglass substrates for high adhesion.

An active matrix light-emitting device can be obtained in the abovemanner.

The structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 5

In this embodiment, examples of a variety of electronic devices that arecompleted using a light-emitting device will be described with referenceto FIGS. 5A to 5D. The light-emitting device is fabricated using thelight-emitting element of one embodiment of the present invention.

Examples of electronic devices including the light-emitting deviceinclude television devices (also referred to as TV or televisionreceivers), monitors for computers and the like, cameras such as digitalcameras and digital video cameras, digital photo frames, mobile phones(also referred to as cellular phones or portable telephone devices),portable game machines, portable information terminals, audio playbackdevices, and large game machines such as pachinko machines. Specificexamples of the electronic devices are illustrated in FIGS. 5A to 5D.

FIG. 5A illustrates an example of a television device. In the televisiondevice 7100, a display portion 7103 is incorporated in a housing 7101.Images can be displayed on the display portion 7103, and thelight-emitting device can be used for the display portion 7103. Inaddition, here, the housing 7101 is supported by a stand 7105

The television device 7100 can be operated by an operation switch of thehousing 7101 or a separate remote controller 7110. With operation keys7109 of the remote controller 7110, channels and volume can becontrolled and images displayed on the display portion 7103 can becontrolled. Furthermore, the remote controller 7110 may be provided witha display portion 7107 for displaying data output from the remotecontroller 7110.

Note that the television device 7100 is provided with a receiver, amodem, and the like. With the use of the receiver, general televisionbroadcasts can be received. Moreover, when the television device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

FIG. 5B illustrates a computer, which includes a main body 7201, ahousing 7202, a display portion 7203, a keyboard 7204, an externalconnection port 7205, a pointing device 7206, and the like. Note thatthis computer can be manufactured using the light-emitting device forthe display portion 7203.

FIG. 5C illustrates a smart watch, which includes a housing 7302, adisplay panel 7304, operation buttons 7311 and 7312, a connectionterminal 7313, a band 7321, a clasp 7322, and the like.

The display panel 7304 mounted in the housing 7302 serving as a bezelincludes a non-rectangular display region. The display panel 7304 candisplay an icon 7305 indicating time, another icon 7306, and the like.

The smart watch illustrated in FIG. 5C can have a variety of functions,for example, a function of displaying a variety of information (e.g., astill image, a moving image, and a text image) on a display portion, atouch panel function, a function of displaying a calendar, date, time,and the like, a function of controlling processing with a variety ofsoftware (programs), a wireless communication function, a function ofbeing connected to a variety of computer networks with a wirelesscommunication function, a function of transmitting and receiving avariety of data with a wireless communication function, and a functionof reading program or data stored in a recording medium and displayingthe program or data on a display portion.

The housing 7302 can include a speaker, a sensor (a sensor having afunction of measuring force, displacement, position, speed,acceleration, angular velocity, rotational frequency, distance, light,liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radiation,flow rate, humidity, gradient, oscillation, odor, or infrared rays), amicrophone, and the like. Note that the smart watch can be manufacturedusing the light-emitting device for the display panel 7304.

FIG. 5D illustrates an example of a mobile phone (e.g., smartphone). Amobile phone 7400 includes a housing 7401 provided with a displayportion 7402, a microphone 7406, a speaker 7405, a camera 7407, anexternal connection portion 7404, an operation button 7403, and thelike. In the case where the light-emitting element of one embodiment ofthe present invention is formed over a flexible substrate, thelight-emitting element can be used for the display portion 7402 having acurved surface as illustrated in FIG. 5D.

When the display portion 7402 of the mobile phone 7400 illustrated inFIG. 5D is touched with a finger or the like, data can be input to themobile phone 7400. In addition, operations such as making a call andcomposing an e-mail can be performed by touch on the display portion7402 with a finger or the like.

There are mainly three screen modes of the display portion 7402. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode in which two modes of the display modeand the input mode are combined.

For example, in the case of making a call or creating e-mail, acharacter input mode mainly for inputting characters is selected for thedisplay portion 7402 so that characters displayed on the screen can beinput. In this case, it is preferable to display a keyboard or numberbuttons on almost the entire screen of the display portion 7402.

When a detection device such as a gyro sensor or an acceleration sensoris provided inside the mobile phone 7400, display on the screen of thedisplay portion 7402 can be automatically changed by determining theorientation of the mobile phone 7400 (whether the mobile phone is placedhorizontally or vertically).

The screen modes are changed by touch on the display portion 7402 oroperation with the button 7403 of the housing 7401. The screen modes canbe switched depending on the kind of images displayed on the displayportion 7402. For example, when a signal of an image displayed on thedisplay portion is a signal of moving image data, the screen mode isswitched to the display mode. When the signal is a signal of text data,the screen mode is switched to the input mode.

Moreover, in the input mode, when input by touching the display portion7402 is not performed for a certain period while a signal detected by anoptical sensor in the display portion 7402 is detected, the screen modemay be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 7402 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken by touchon the display portion 7402 with the palm or the finger, wherebypersonal authentication can be performed. In addition, when a backlightor a sensing light source that emits near-infrared light is provided inthe display portion, an image of a finger vein, a palm vein, or the likecan be taken.

Furthermore, the light-emitting device can be used for a mobile phonehaving a structure illustrated in FIG. 5D′-1 or FIG. 5D′-2, which isanother structure of the mobile phone (e.g., smartphone).

Note that in the case of the structure illustrated in FIG. 5D′-1 or FIG.5D′-2, text data, image data, or the like can be displayed on secondscreens 7502(1) and 7502(2) of housings 7500(1) and 7500(2) as well asfirst screens 7501(1) and 7501(2). Such a structure enables a user toeasily see text data, image data, or the like displayed on the secondscreens 7502(1) and 7502(2) while the mobile phone is placed in user'sbreast pocket.

As described above, the electronic devices can be obtained using thelight-emitting device that includes the light-emitting element of oneembodiment of the present invention. Note that the light-emitting devicecan be used for electronic devices in a variety of fields without beinglimited to the electronic devices described in this embodiment.

The structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 6

In this embodiment, examples of lighting devices will be described withreference to FIG. 6. Each of the lighting devices uses a light-emittingdevice including the light-emitting element of one embodiment of thepresent invention.

FIG. 6 illustrates an example in which the light-emitting device is usedas an indoor lighting device 8001. Since the light-emitting device canhave a large area, it can be used for a lighting device having a largearea. In addition, a lighting device 8002 in which a light-emittingregion has a curved surface can also be obtained with the use of ahousing with a curved surface. A light-emitting element included in thelight-emitting device described in this embodiment is in a thin filmform, which allows the housing to be designed more freely. Thus, thelighting device can be elaborately designed in a variety of ways. Inaddition, a wall of the room may be provided with a large-sized lightingdevice 8003.

When the light-emitting device is used for a surface of a table, alighting device 8004 that has a function as a table can be obtained.When the light-emitting device is used as part of other furniture, alighting device that functions as the furniture can be obtained.

As described above, a variety of lighting devices that include thelight-emitting device can be obtained. Note that these lighting devicesare also embodiments of the present invention.

The structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

EXAMPLE

In this example, a light-emitting element 1 and a light-emitting element2 of one embodiment of the present invention, and a comparativelight-emitting element 3 and a comparative light-emitting element 4 forcomparison were fabricated. The element structures will be described indetail with reference to FIG. 7. A light-emitting element in thisexample has a structure combining the tandem structure described inEmbodiment 2 and the microcavity structure described in Embodiment 3.Chemical formulae of materials used in this example are shown below.

<<Fabrication of Light-Emitting Elements 1 and 2, and ComparativeLight-Emitting Elements 3 and 4>22

In this example, the light-emitting element 1 and the comparativelight-emitting element 3 which emit red light are shown on the left ofFIG. 7, and the light-emitting element 2 and the comparativelight-emitting element 4 which emit green light are shown on the rightof FIG. 7. Note that all these light-emitting elements have a structurein which light is emitted from the second electrode 4003 side.

In the light-emitting elements 1 and 2, the same material is used for alight-emitting layer (4013(b 2)) in a second EL layer 4002 b. Also, thematerial for the light-emitting layer (4013(b 2)) in the second EL layer4002 b is the same in the comparative light-emitting elements 3 and 4,but is different from the material used in the light-emitting elements 1and 2. The light-emitting element 1 and the comparative light-emittingelement 3 shown on the left of FIG. 7 are optically adjusted so that redlight emission is obtained. The light-emitting element 2 and thecomparative light-emitting element 4 shown on the right of FIG. 7 areoptically adjusted so that green light emission is obtained. Therefore,a first electrode 4001 on the left of FIG. 7 has a structure differentfrom that of a first electrode 4101 on the right of FIG. 7. Note thatother common components are denoted by the same reference numerals inFIG. 7.

For the light-emitting elements 1 and 2, first, an alloy film ofaluminum (Al), nickel (Ni), and lanthanum (La) (Al—Ni—La alloy film)with a thickness of 200 nm was deposited over a glass substrate 4000 bya sputtering method, a film of Ti with a thickness of 6 nm was depositedby a sputtering method, and then a film of indium tin oxide containingsilicon oxide (ITSO) was deposited by a sputtering method. For thecomparative light-emitting elements 3 and 4, an alloy film of aluminum(Al) and titanium (Ti) (Al—Ti alloy film) with a thickness of 200 nm wasdeposited over the glass substrate 4000 by a sputtering method, a filmof Ti with a thickness of 6 nm was deposited by a sputtering method, andthen a film of indium tin oxide containing silicon oxide (ITSO) wasdeposited by a sputtering method. In that case, the thickness of theITSO film was 75 nm in the light-emitting element 1, 40 nm in thelight-emitting element 2, 80 nm in the comparative light-emittingelement 3, and 40 nm in the comparative light-emitting element 4. Thus,the first electrodes 4001 and 4101 serving as anodes of thelight-emitting elements 1 and 2 and the comparative light-emittingelements 3 and 4 were formed. At this time, the films of Ti arepartially or entirely oxidized and contain titanium oxide. Note that theelectrode area was 2 mm×2 mm.

Then, as pretreatment for forming the light-emitting elements 1 and 2and the comparative light-emitting elements 3 and 4 over the substrate4000, UV ozone treatment was performed for 370 seconds after washing ofa surface of the substrate with water and 1-hour baking at 200° C.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 60 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate 4000 was cooled down for about 30 minutes.

Next, the substrate 4000 was fixed to a holder provided in the vacuumevaporation apparatus so that a surface of the substrate 4000 providedwith the first electrodes (4001 and 4101) was directed downward. In thisexample, by a vacuum evaporation method, a first hole-injection layer4011 a, a first hole-transport layer 4012 a, a light-emitting layer (A)(4013 a), a first electron-transport layer 4014 a, and a firstelectron-injection layer 4015 a, which are included in a first EL layer4002 a, were sequentially formed, and then a charge-generation layer4004 was formed. After that, a second hole-injection layer 4011 b, asecond hole-transport layer 4012 b, a light-emitting layer (B) (4013(b1) and 4013(b 2)), a second electron-transport layer 4014 b, and asecond electron-injection layer 4015 b, which are included in a secondEL layer 4002 b, were sequentially formed.

The pressure in the vacuum evaporation apparatus was reduced to 10⁻⁴ Pa.Then, for the light-emitting elements 1 and 2,9-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]phenanthrene (abbreviation:PcPPn) and molybdenum oxide were co-evaporated at a mass ratio of 1:0.5(PcPPn: molybdenum oxide), whereby the first hole-injection layer 4011 awas formed over the first electrodes (4001 and 4101). Note that thethickness of the first hole-injection layer 4011 a was 10 nm in each ofthe light-emitting elements 1 and 2. For the comparative light-emittingelements 3 and 4, the first hole-injection layer 4011 a was formed byco-evaporation of9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation:PCzPA), and molybdenum oxide at a mass ratio of 1:0.5. Note that thethickness of the first hole-injection layer 4011 a was 10 nm in thecomparative light-emitting element 3 and 13 nm in the comparativelight-emitting element 4.

Then, the first hole-transport layer 4012 a was formed by evaporation ofPcPPn (abbreviation). The thickness of the first hole-transport layer4012 a was 15 nm in each of the light-emitting elements 1 and 2, and 20nm in each of the comparative light-emitting elements 3 and 4.

Next, a light-emitting layer (A) 4013 a was formed over the firsthole-transport layer 4012 a. The light-emitting layer (A) 4013 a wasformed by co-evaporation of9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation: CzPA)andN,N′-bis(3-methylphenye-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine(abbreviation: 1,6mMemFLPAPrn) at a mass ratio of 1:0.05 (CzPA:1,6mMemFLPAPm). The thickness of the light-emitting layer (A) 4013 a was25 nm in each of the light-emitting elements 1 and 2, and 30 nm in eachof the comparative light-emitting elements 3 and 4.

After that, the first electron-transport layer 4014 a was formed overthe light-emitting layer (A) 4013 a by evaporation of a 5-nm-thick filmof CzPA (abbreviation) and then evaporation of a 15-nm-thick film ofbathophenanthroline (abbreviation: BPhen). In addition, the firstelectron-injection layer 4015 a was formed by evaporation of a 0.1-nmthick film of lithium oxide (Li₂O) over the first electron-transportlayer 4014 a.

Then, the charge-generation layer 4004 was formed over the firstelectron-injection layer 4015 a by evaporation of a 2-nm-thick film ofcopper phthalocyanine (abbreviation: CuPc).

Next, for the light-emitting elements 1 and 2,1,3,5-tri(dibenzothiophen-4-yl)benzene (abbreviation: DBT3P-II) andmolybdenum oxide were co-evaporated at a mass ratio of 1:0.5 (DBT3P-II:molybdenum oxide), whereby the second hole-injection layer 4011 b with athickness of 12.5 nm was formed over the charge-generation layer 4004.For the comparative light-emitting elements 3 and 4,9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation:PCzPA) and molybdenum oxide were co-evaporated at a mass ratio of 1:0.5(PCzPA: molybdenum oxide), whereby the second hole-injection layer 4011b with a thickness of 13 nm was formed.

Then, the second hole-transport layer 4012 b was formed by evaporationof a 20-nm-thick film of4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP).

After that, the light-emitting layer (B) (a first light-emitting layer4013(b 1) and a second light-emitting layer 4013(b 2)) was formed overthe second hole-transport layer 4012 b.

For the light-emitting elements 1 and 2, the first light-emitting layer4013(b 1) was formed to a thickness of 20 nm by co-evaporation of2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: PCBBiF), and(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(tBuppm)2(acac)]) at a mass ratio of 0.8:0.2:0.06(2mDBTBPDBq-II: PCBBiF: [Ir(tBuppm)2(acac)]). For the comparativelight-emitting elements 3 and 4, the first light-emitting layer 4013(b1) was formed to a thickness of 20 nm by co-evaporation of2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB), and(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(tBuppm)₂(acac)]) at a mass ratio of 0.8:0.2:0.06(2mDBTBPDBq-II: PCBNBB: [ft(tBuppm)₂(acac)]).

For the light-emitting elements 1 and 2, the second light-emitting layer4013(b 2) was formed to a thickness of 20 nm by co-evaporation of2mDBTBPDBq-II (abbreviation) and

-   bis{4,6-dimethyl-2-[5-(2,6-dimethylphenyl)-3-(3,5-dimethylphenyl)-2-pyrazinyl-κN]phenyl-κC}(2,4-pentanedionato-κk²O,O′)iridium(III)    (abbreviation: [Ir(dmdppr-dmp)₂(acac)]) at a mass ratio of 1:0.06    (2mDBTBPDBq-II: [Ir(dmdppr-dmp)₂(acac)]). For the comparative    light-emitting elements 3 and 4, the second light-emitting layer    4013(b 2) was formed to a thickness of 20 nm by co-evaporation of    2mDBTBPDBq-II and-   bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)    (abbreviation: [Ir(tppr)₂(dpm)]at a mass ratio of 1:0.06    (2mDBTBPDBq-II: [Ir(tppr)₂(dpm)]).

Next, the second electron-transport layer 4014 b was formed over thelight-emitting layer (B) 4013 b.

For the light-emitting elements 1 and 2, the second electron-transportlayer 4014 b was formed by evaporation of a 35-nm-thick film of2mDBTBPDBq-II (abbreviation) and then evaporation of a 15-nm-thick filmof BPhen (abbreviation). For the comparative light-emitting elements 3and 4, the second electron-transport layer 4014 b was formed byevaporation of a 15-nm-thick film of 2mDBTBPDBq-II (abbreviation) andthen evaporation of a 15-nm-thick film of BPhen (abbreviation).

Furthermore, the second electron-injection layer 4015 b was formed overthe second electron-transport layer 4014 b by evaporation of a1-nm-thick film of lithium fluoride (LiF).

Finally, the second electrode 4003 serving as a cathode was formed overthe second electron-injection layer 4015 b. The second electrode 4003was obtained by forming a 15-nm-thick film of silver (Ag) and magnesium(Mg) by co-evaporation at a mass ratio of 1:0.1, and then forming a70-nm-thick film of indium tin oxide (ITO) by a sputtering method. Notethat in all the above evaporation steps, evaporation was performed by aresistance heating method.

Table 1 shows the structures of the light-emitting elements 1 and 2 andthe comparative light-emitting elements 3 and 4 obtained through theabove steps.

TABLE 1 First hole- First hole- Light- First electron- Numerals Firstelectrode injection layer transport layer emitting layer (A) transportlayer in FIG. 7 4001, 4101 4011a 4012a 4013a 4014a Light-emittingAl—Ni—La\Ti ITSO PCPPn:MoOx PCPPn *1 CzPA BPhen element 1 (R) (200 nm\6nm) (75 nm) (1:0.5 10 nm) (15 nm) (5 nm) (15 nm) Light-emitting ITSOPCPPn:MoOx element 2 (G) (40 nm) (1:0.5 10 nm) Comparative Al—Ti\Ti ITSOPCzPA:MoOx PCPPn *2 light-emitting (200 nm\6 nm) (80 nm) (1:0.5 10 nm)(20 nm) element 3 (R) Comparative ITSO PCzPA:MoOx light-emitting (40 nm)(1:0.5 13 nm) element 4 (G) Light-emitting layer (B) First electron-Charge Second hole- Second hole- First light- Second light- injectionlayer generation layer injection layer transport layer emitting layeremitting layer 4015a 4004 4011b 4012b 4013 (b1) 4013 (b2) (Remarks) Li₂OCuPc DBT3P-II:MoOx BPAFLP *3 *5 Light-emitting (0.1 nm) (2 nm) (1:0.512.5 nm) (20 nm) element 1 (R) Light-emitting element 2 (G) PCzPA:MoOx*4 *6 Comparative (1:0.5 13 nm) light-emitting element 3 (R) Comparativelight-emitting element 4 (G) Second electron-transport layer Secondelectron-injection layer Second electrode 4014b 4015b 4003 CF2mDBTBPDBq-II BPhen LiF Ag:Mg ITO R (2.36 μm) Light-emitting (35 nm) (15nm) (1 nm) (1:0.1 15 nm) (70 nm) element 1 (R) G (1.29 μm)Light-emitting element 2 (G) 2mDBTBPDBq-II R (2.36 μm) Comparative (15nm) light-emitting element 3 (R) G (1.29 μm) Comparative light-emittingelement 4 (G) *1 CzPA:1,6mMemFLPAPrn (1:0.05 25 nm) *2CzPA:1,6mMemFLPAPrn (1:0.05 30 nm) *32mDBTBPDBq-II:PCBBiF:[Ir(tBuppm)₂(acac)] (0.8:0.2:0.06 20 nm) *42mDBTBPDBq-II:PCBNBB:[Ir(tBuppm)₂(acac)] (0.8:0.2:0.06 20 nm) *52mDBTBPDBq-II:[Ir(dmdppr-dmp)₂(acac)] (1:0.06 20 nm) *62mDBTBPDBq-II:[Ir(tppr)₂(dpm)] (1:0.06 20 nm)

As shown in Table 1, a red coloring layer (R) is formed on a countersubstrate of each of the light-emitting element 1 and the comparativelight-emitting element 3, and a green coloring layer (G) is formed on acounter substrate of the light-emitting element 2 and the comparativelight-emitting element 4. The fabricated light-emitting elements 1 and 2and comparative light-emitting elements 3 and 4 were sealed by beingbonded to these counter substrates in a glove box containing a nitrogenatmosphere so as not to be exposed to the air (specifically, a sealantwas applied on the outer edge of the element, and irradiation withultraviolet light with a wavelength of 365 nm at 6 J/cm² and heattreatment at 80° C. for 1 hour were performed for sealing).

<<Operation Characteristics of Light-emitting Elements 1 and 2 andComparative Light-emitting Elements 3 and 4>>

The operation characteristics of the fabricated light-emitting elements1 and 2 and comparative light-emitting elements 3 and 4 were measured.Note that the measurement was carried out at room temperature (under anatmosphere in which the temperature was kept at 25° C.).

First, FIG. 8 shows the luminance-current efficiency characteristics ofthe light-emitting elements 1 and 2 and the comparative light-emittingelements 3 and 4. In FIG. 8, the vertical axis represents currentefficiency (cd/A) and the horizontal axis represents luminance (cd/m²).

Table 2 shows the initial values of the main characteristics of thelight-emitting elements 1 and 2 and the comparative light-emittingelements 3 and 4 at a luminance of approximately 1000 cd/m².

TABLE 2 Current Current Power Voltage Current density ChromaticityLuminance efficiency efficiency (V) (mA) (mA/cm²) (x, y) (cd/m²) (cd/A)(lm/W) Light-emitting 7.2 0.2  5.5 (0.67, 0.33)  950 17 7.5 element 1(R) Light-emitting 6.4 0.067 1.7 (0.27, 0.71) 1000 61 30   element 2 (G)Comparative 6.7 0.3  8.1 (0.67, 0.33)  950 12 5.5 light-emitting element3 (R) Comparative 6.2 0.13  3.3 (0.26, 0.72) 1100 33 17   light-emittingelement 4 (G)

The above results are summarized as follows. The light-emitting element1 and the comparative light-emitting element 3 fabricated in thisexample use different materials for the second light-emitting layer4013(b 2) in the light-emitting layer (B) (4013 b), though both of theelements 1 and 3 emit red (R) light. Specifically, in the light-emittingelement 1, a host material is 2mDBTBPDBq-II and a guest material is[Ir(dmdppr-dmp)₂(acac)]. In contrast, in the comparative light-emittingelement 3, a host material is the same as that in the light-emittingelement 1, but a guest material is [Ir(tppr)₂(dpm)]. Note that as theresult of cyclic voltammetry (CV) measurement, the LUMO of the hostmaterial (2mDBTBPDBq-II) was −2.94 eV, the LUMO of the guest material([Ir(dmdppr-dmp)₂(acac)]) in the light-emitting element 1 was −2.91 eV,and the LUMO of the guest material ([Ir(tppr)₂(dpm)]) in the comparativelight-emitting element 3 was −3.05 eV. Thus, the LUMO level of the guestmaterial is in the range of ±0.1 eV of the LUMO level of the hostmaterial in the case of the light-emitting element 1, while it is not inthe range of ±0.1 eV in the case of the comparative light-emittingelement 3. Such a difference in LUMO level probably causes a differencein the current efficiency characteristics between the light-emittingelement 1 and the comparative light-emitting element 3.

In comparison between the light-emitting element 2 and the comparativelight-emitting element 4, both of which emit green (G) light, thelight-emitting element 2 exhibits higher current efficiency as shown inTable 2. Note that the light-emitting elements 1 and 2 include the sameguest material exhibiting red emission in the second light-emittinglayer 4013(b 2) in the light-emitting layer (B) (4013 b), and the sameguest material exhibiting green emission in the first light-emittinglayer 4013(b 1). The reason why the current efficiency of thelight-emitting element 2 is higher than that of the comparativelight-emitting element 4 is probably as follows. The host material andthe guest material used for the second light-emitting layer 4013(b 2)have the aforementioned relationship of LUMO level, so that the currentefficiency of the red-emitting element increases. This increases therecombination efficiency of carriers in the first light-emitting layer4013(b 1) stacked on the second light-emitting layer 4013(b 2). Hence,the current efficiency of the green-emitting elements also increases.

As described above, when a red-emitting element and a green-emittingelement are obtained by combining a coloring layer (color filter) with awhite-emitting element, the current efficiency of not only thered-emitting element but also the green-emitting element is increased bychanging only a red-emitting material, which could not contribute togreen light emission. Such a phenomenon cannot be expected in general.In other words, this phenomenon is a significant effect of the structureof the present invention.

An appropriate combination of these light-emitting elements willtherefore offer a full-color or white light-emitting device with a highcurrent efficiency.

FIG. 9 shows the emission spectra of the light-emitting elements 1 and 2and the comparative light-emitting elements 3 and 4, through which acurrent flows at a current density of 2.5 mA/cm². As shown in FIG. 9,the emission spectra of the light-emitting element 1(R) and thecomparative light-emitting element 3(R) have peaks at around 611 nm, andthe emission spectra of the light-emitting element 2(G) and thecomparative light-emitting element 4(G) have peaks at around 534 nm. Theemission of each light-emitting element is derived from a phosphorescentorganometallic iridium complex included in the light-emitting layer.

Note that in comparison between the light-emitting element 1 and thecomparative light-emitting element 3, both of which emit red light, thespectrum of the light-emitting element 1 is narrower than that of thecomparative light-emitting element 3; and in comparison between thelight-emitting element 2 and the comparative light-emitting element 4,both of which emit green light, the spectrum of the light-emittingelement 2 is narrower than that of the comparative light-emittingelement 4. Such narrowing of the spectra also indicates that the currentefficiency of the light-emitting elements 1 and 2 become higher thanthat of the comparative light-emitting elements 3 and 4.

EXPLANATION OF REFERENCE

-   101: anode, 102: cathode, 103: EL layer, 104: hole-injection layer,    105: hole-transport layer, 106: light-emitting layer, 106 a: first    light-emitting layer, 106 b: second light-emitting layer, 107:    electron-transport layer, 108: electron-injection layer, 109: guest    material, 109 a: first light-emitting material, 109 b: second    light-emitting material, 110: host material, 201: first electrode,    202(1): first EL layer, 202(2): second EL layer, 202(n-1): (n-1)th    EL layer, 202(n): n-th EL layer, 204: second electrode, 205:    charge-generation layer, 205(1): first charge-generation layer,    205(2): second charge-generation layer, 205(n-2): (n-2)th    charge-generation layer, 205(n-1): (n-1)th charge-generation layer,    301: reflective electrode, 302: transflective electrode, 303 a:    first transparent conductive layer, 303 b: second transparent    conductive layer, 304P: first light-emitting layer, 304Q: second    light-emitting layer, 305: EL layer, 310P: first light-emitting    element, 310Q: second light-emitting element, 401: element    substrate, 402: pixel portion, 403: driver circuit portion (source    line driver circuit), 404 a, 404 b: driver circuit portion (gate    line driver circuit), 405: sealant, 406: sealing substrate, 407:    wiring, 408: FPC (flexible printed circuit), 409: FET, 410: FET,    411: switching FET, 412: current control FET, 413: first electrode    (anode), 414: insulator, 415: EL layer, 416: second electrode    (cathode), 417: light-emitting element, 418: space, 4000: substrate,    4001: electrode, 4101: electrode, 4002 a: EL layer, 4002 b: EL    layer, 4003: electrode, 4004: charge-generation layer, 4011 a:    hole-injection layer, 4011 b: hole-injection layer, 4012 a:    hole-transport layer, 4012 b: hole-transport layer, 4013 a:    light-emitting layer (A), 4013 b: light-emitting layer (B), 4013(b    1): light-emitting layer (B); first light-emitting layer, 4013(b 2):    light-emitting layer (B); second light-emitting layer, 4014 a:    electron-transport layer, 4014 b: electron-transport layer, 4015 a:    electron-injection layer, 4015 b: electron-injection layer, 7100:    television device, 7101: housing, 7103: display portion, 7105:    stand, 7107: display portion, 7109: operation key, 7110: remote    controller, 7201: main body, 7202: housing, 7203: display portion,    7204: keyboard, 7205: external connection port, 7206: pointing    device, 7302: housing, 7304: display panel, 7305: icon indicating    time, 7306: another icon, 7311: operation button, 7312: operation    button, 7313: connection terminal, 7321: band, 7322: clasp, 7400:    mobile phone, 7401: housing, 7402: display portion, 7403: button,    7404: external connection portion, 7405: speaker, 7406: microphone,    7407: camera, 7500(1), 7500(2): housing, 7501(1), 7501(2): first    screen, 7502(1), 7502(2): second screen, 8001: lighting device,    8002: lighting device, 8003: lighting device, 8004: lighting device.

This application is based on Japanese Patent Application serial No.2014-031792 filed with Japan Patent Office on Feb. 21, 2014, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A light-emitting element comprising: afirst EL layer between an anode and a cathode; and a light-emittinglayer between the anode and the cathode, wherein the first EL layerincludes a first light-emitting layer and a second light-emitting layer,wherein the first light-emitting layer is located between the cathodeand the second light-emitting layer, wherein the first light-emittinglayer is in contact with the second light-emitting layer, wherein thefirst light-emitting layer includes a first host material and a firstguest material, wherein the second light-emitting layer includes asecond host material and a second guest material, wherein an emissionpeak of the second guest material is at a shorter wavelength than anemission peak of the first guest material, wherein a lowest unoccupiedmolecular orbital level of the first guest material is in a range ofhigher than 0 eV and lower than 0.1 eV of a lowest unoccupied molecularorbital level of the first host material, wherein each of the firstguest material and the second guest material is a phosphorescentorganometallic iridium complex, and wherein the first guest material isdifferent from the second guest material.
 2. The light-emitting elementaccording to claim 1, further comprising: a second EL layer between theanode and the cathode; and a charge-generation layer between the firstEL layer and the second EL layer.
 3. The light-emitting elementaccording to claim 2, wherein the first EL layer is located between thecharge-generation layer and the cathode.
 4. The light-emitting elementaccording to claim 2, wherein the first EL layer is located between thecharge-generation layer and the anode.
 5. The light-emitting elementaccording to claim 1, further comprising a third light-emitting layer,wherein the third light-emitting layer is located between the firstlight-emitting layer and the cathode, wherein the third light-emittinglayer is in contact with the first light-emitting layer, and wherein thesecond light-emitting layer and the third light-emitting layer includethe same material.
 6. The light-emitting element according to claim 1,wherein emission in the first light-emitting layer has a peak at awavelength from 560 nm to 700 nm, and wherein emission in the secondlight-emitting layer has a peak at a wavelength from 500 nm to 560 nm.7. A light-emitting device comprising: the light-emitting elementaccording to claim 1; and a flexible printed circuit.
 8. An electronicdevice comprising: the light-emitting device according to claim 7; andan operation key, a speaker, a microphone, or an external connectionportion.
 9. A lighting device comprising: the light-emitting deviceaccording to claim 7; and a housing.
 10. The light-emitting elementaccording to claim 1, wherein the first guest material is anorganometallic iridium complex in which a ligand is a phenylpyrazinederivative having an alkyl group.
 11. The light-emitting elementaccording to claim 1, wherein the first guest material is anorganometallic iridium complex in which a ligand is a phenylpyrazinederivative having an alkyl group, and wherein the first host material isan electron-transport host material including a quinoxaline derivative.12. The light-emitting element according to claim 1, wherein the firstguest material exhibits red emission, and wherein the second guestmaterial exhibits green emission.
 13. A light-emitting elementcomprising: a first EL layer between an anode and a cathode; and alight-emitting layer between the anode and the cathode, wherein thefirst EL layer includes a first light-emitting layer and a secondlight-emitting layer, wherein the first light-emitting layer is locatedbetween the cathode and the second light-emitting layer, wherein thefirst light-emitting layer is in contact with the second light-emittinglayer, wherein the first light-emitting layer includes anelectron-transport material, and a hole-transport material, and a firstguest material, wherein the second light-emitting layer includes a hostmaterial and a second guest material, wherein an emission peak of thesecond guest material is shorter than an emission peak of the firstguest material, wherein a lowest unoccupied molecular orbital level ofthe first guest material is in a range of higher than 0 eV and lowerthan 0.1 eV of a lowest unoccupied molecular orbital level of one of theelectron-transport material and the hole-transport material, whereineach of the first guest material and the second guest material is aphosphorescent organometallic iridium complex, and wherein the firstguest material is different from the second guest material.
 14. Thelight-emitting element according to claim 13, further comprising: asecond EL layer between the anode and the cathode; and acharge-generation layer between the first EL layer and the second ELlayer.
 15. The light-emitting element according to claim 14, wherein thefirst EL layer is located between the charge-generation layer and thecathode.
 16. The light-emitting element according to claim 14, whereinthe first EL layer is located between the charge-generation layer andthe anode.
 17. The light-emitting element according to claim 13, furthercomprising a third light-emitting layer, wherein the thirdlight-emitting layer is located between the first light-emitting layerand the cathode, wherein the third light-emitting layer is in contactwith the first light-emitting layer, and wherein the secondlight-emitting layer and the third light-emitting layer include the samematerial.
 18. The light-emitting element according to claim 13, whereinemission in the first light-emitting layer has a peak at a wavelengthfrom 560 nm to 700 nm, and wherein emission in the second light-emittinglayer has a peak at a wavelength from 500 nm to 560 nm.
 19. Alight-emitting device comprising: the light-emitting element accordingto claim 13; and a flexible printed circuit.
 20. An electronic devicecomprising: the light-emitting device according to claim 19; and anoperation key, a speaker, a microphone, or an external connectionportion.
 21. A lighting device comprising: the light-emitting deviceaccording to claim 19; and a housing.
 22. The light-emitting elementaccording to claim 13, wherein the first guest material is anorganometallic iridium complex in which a ligand is a phenylpyrazinederivative having an alkyl group.
 23. The light-emitting elementaccording to claim 13, wherein the first guest material is anorganometallic iridium complex in which a ligand is a phenylpyrazinederivative having an alkyl group, and wherein the electron-transportmaterial includes a quinoxaline derivative.
 24. The light-emittingelement according to claim 13, wherein the first guest material exhibitsred emission, and wherein the second guest material exhibits greenemission.